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Old 18th March 2010, 10:08 PM   #1
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Question transformer question

I am building a tube preamp from Rozenblit schematic; just got in the transformers, a Triad F4-120 and Triad F5-12. The spec sheets indicate 115v input; my mains are 122v. Output on the 120 is around 156v and output from the 12 is about 16v. Is this all because of the higher mains voltage? Should I lower the mains to get correct voltage, or lower the output? thanks
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Old 18th March 2010, 10:44 PM   #2
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There are 2 reasons for the higher voltage. If you place the transformer under a load
your voltages should come down to about the expected 120v and 12v. Small transformers like yours have to use a lot of verry fine wire so the load regulation isn't verry good. Try placing a load on that 12v winding to draw the rated current and you
will probably see that 16v drop to ~12v.
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Old 18th March 2010, 10:51 PM   #3
tomchr is offline tomchr  United States
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I'm guessing you are measuring the transformer without load. The transformer should be rated at full load, so you will get a higher voltage if you measure without load.

The mains voltage will vary depending on a lot of factors. Time of day, time of year, your location relative to the substation, etc., etc. It looks like your mains voltage is within the 120 V +/-5 % spec. The voltage at my house measured 123.5 V the other night when I was testing my variac...

Bottom line: Don't worry about it.

~Tom
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Old 19th March 2010, 12:14 AM   #4
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Default (load)ed question

Thanks guys, that makes sense. So, when my power supply is built, I can't really test it without a load, which in this case is supposed to be a tube...is there some dummy load you use to test transformers/power supplies? thanks
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Old 19th March 2010, 02:37 PM   #5
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Dummy load is a bad choice. Dummy loads will have to draw the intended current and thus must be of any appropriate power rating. E.g. If you wish to test your 120V 0.2A (for example) tap under a load you`ll have to get a resistor with the parametres:

R = U/I = 120*5 = 600 Ohm, and
P = U*I = 120/5 = 24 W

To have some safety headroom, that will be a 30W resistor which is a rather BIG one.
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